It was noted that epitaxial layers frequently contained a high density of threading dislocations, which could exceed values of 109/cm2 for ZnO. In order to study the electrical activity of single dislocations, off-axis electron holography in a transmission electron microscope was used. The electrostatic potential in the vicinity of charged dislocations was deduced from the reconstructed phase of the electron wave, which also provided a guide to the charge density at the dislocation. On the basis of the electrostatic potential for a screened line charge, a refined model was proposed in which a charge density with a finite spatial distribution at the dislocation core was assumed. Upon comparing the measured and theoretical potential, charge densities of between 5 x 1019 and 5 x 1020/cm3 in cylinders - with radii up to 5nm - around the dislocation lines were found. An important aspect was an analysis of dislocations in cross-sectional transmission electron microscopy samples, where the dislocation lines were oriented perpendicularly to the electron beam. Electrostatic potentials due to piezoelectric charges in the dislocation strain field were considered and were found to be insignificant with respect to dislocation charges.

Probing the Electrostatic Potential of Charged Dislocations in n-GaN and n-ZnO Epilayers by Transmission Electron Holography. Müller, E., Gerthsen, D., Brückner, P., Scholz, F., Gruber, T., Waag, A.: Physical Review B, 2006, 73[24], 245316